At the production of expandable microspheres a liquid suspension of thermoplastic partices is obtained and the particles contain an encapsulated liquid blowing agent. The unexpanded particles can be used as such, they can for example be mixed into different products for later expansion by heating. However, the expansion can also be carried out on an industrial scale before the spheres are mixed into any product or used in another way.
The actual expansion process is, however, not free from problems. It is desirable to use a high temperature to get a uniform and rapid expansion as well as maximum decrease in density due to great gas expansion and softened thermoplastic shell. However, the softening of the thermoplastic material also leads to a risk for particle shell weakening and to a mutual particle agglomeration. The agglomeration tendency increases with the time the particles are in contact with each other in a heated condition. If large suspension media amounts are used to avoid agglomeration there will instead be problems with the moisture content of the final product. Problems with agglomeration can also arise at heating in connection with drying of the particles, for example when the suspension liquid is dried off. If the microspheres are rapidly cooled with water to avoid agglomeration a conflict will arise with the need to avoid substantial drying.
Further problems arise with the application of heat in connection with the expansion. The expanded particles have a considerably higher thermal insulating effect than the unexpanded particles and this can make a complete and uniform expansion of the entire quantity of microspheres more difficult. Also the drying of microspheres by means of heating is more difficult for the same reasons. If attempts are made to avoid these problems by finely atomizing the particles there will be economical problems due to the necessity for larger equipment volumes.
Several different methods to carry out the expansion have been disclosed. The U.S. Pat. No. 3,611,583 suggests that the slurry of microspheres be spread out in a thin layer on a belt with subsequent application of heat. Even if the layer is very thin it cannot be avoided that heat supply to the central parts of the mass is less good than to the parts close to the surface. Further, in this method the particles will be in contact with each other for a fairly long time in heated condition and this gives a high risk of agglomeration. The U.S. Pat. No. 3,779,951 suggests that glycols be used as lubricant for the particles to avoid agglomeration. Besides the fact that in most cases this method gives an addition of an undesirable component to the microspheres, it does not solve the problem of obtaining a uniform supply of heat. According to the U.S. Pat. No. 3,914,360 the microspheres are expanded by being conveyed through a heated static mixer. Although the continuous mixing improves the heat transfer this will not be complete due to the insulating effect of the expanded particles. Further, there is nothing in this process that will prevent a continuous agglomeration between the particles during the expansion phase or in the obtained warm cake of expanded spheres. The Swedish patent application No. 8100181-0 suggests atomization a slurry of microspheres in a hot, inert gas whereby the spheres are dried and then expanded in a dispersed condition forming a free-flowing product. However, this process requires large gas- and apparatus volumes. There is also a commercial method according to which, the slurry is fed to a pipe together with steam whereby the particles expand. The disadvantage of this method is that the expanded particles leaving the pipe will have to be cooled directly with water in order not to agglomerate and this addition of water, in combination with the comparatively low dry content of the feed slurry, results in a final product having unsatisfactory low dry content. If cooling water is not used or is replaced by cooling air or a cooled apparatus mantle a product of fused particles will be obtained.